The present invention relates to a flue gas denitration catalyst for the removal of nitrogen oxides from a flue gas of a combustion furnace such as large-sized boiler for electricity generation; and a preparation process of the catalyst.
A flue gas from a boiler usually contains nitrogen oxides and sulfur oxides. One denitration method for such a flue gas is to add ammonia to the flue gas which passes over a catalyst composed mainly of titanium (Ti), tungsten (W), and vanadium (V) where nitrogen oxides are treated in accordance with the following reaction formula:4NO+4NH3+O2→4N2+6H2O
A flue gas denitration catalyst can ordinarily be prepared by forming, into a monolithic honeycomb shape, a powdery catalyst obtained by supporting tungsten trioxide (WO3) and vanadium pentoxide (V2O5) on a carrier of titanium dioxide (TiO2) by impregnation. This preparation process however involves such a problem that an increase in the amount of V2O5, which is a main active component of the catalyst, improves denitration activity but it simultaneously enhances oxidation of sulfur dioxide which is a reaction as shown in the below-described reaction formula:2SO2+O2→2SO3.
There is therefore proposed a preparation process comprising forming a TiO2 powder into a honeycomb carrier, supporting WO3 on the resulting honeycomb carrier by an impregnation method and then supporting V2O5 on the resulting carrier by a vapor phase method (refer to Japanese Examined Patent Publication No. 6-40957). Compared with the conventional catalyst which is obtained by impregnation and whose V2O5 concentration is uniform even inside the bulk, the catalyst obtained by the above-described process can contain V2O5 thinly and uniformly along the surface of the catalyst at a high concentration so that it is possible to promote the denitration reaction which proceeds sufficiently in the surface layer of the catalyst alone and to prevent the oxidation of SO2 occurring even inside the bulk.
Emission standards of nitrogen oxides are becoming more stringent, and flue gas denitration catalysts have to have higher denitration performance. In addition, in the denitration method employed particularly for the exhaust gas from a coal-fired boiler among various exhaust gases, catalysts capable of suppressing oxidation of SO2 which is a side reaction and having high denitration activity are required. Moreover, in the method as described in the above patent publication, it is difficult to form TiO2 into a monolithic honeycomb, because upon its formation, even if various binders are added to TiO2, they fail to give sufficient strength to the TiO2 carrier.